1. Field of the Invention
The present invention relates to a turbocharger and, more particularly, to a turbocharger including a waste gate structure for controlling a boost pressure.
2. Description of the Prior Art
A conventional turbocharger is disclosed in, for example, Japanese patent application laid-open publication No. 4(1992)-103817. This turbocharger includes a turbine rotor, a turbine housing in which the turbine rotor rotated by exhaust gas is disposed therein and which is provided with an exhaust gas inlet and an exhaust gas outlet, a bearing housing which is fixed to the turbine housing, a shaft which is rotatably supported on the bearing housing via a radial bearing and on one end of which the turbine rotor is fixed thereon, a compressor rotor which is fixed on the other end of the shaft and which is disposed in a compressor housing fixed to the bearing housing, a bypass passage which is formed on the turbine housing so as to communicate between the exhaust gas inlet and the exhaust gas outlet while bypassing the turbine rotor and a waste gate valve which is disposed in the bypass passage and which opens or closes the bypass passage in response to a boost pressure (a pressure of intake air supercharged by the compressor rotor). In this turbocharger, when the boost pressure exceeds a predetermined value, the waste gate valve is opened by a driving mechanism and the exhaust gas inlet communicates with the exhaust gas outlet. Thereby, a part of the exhaust gas is discharged to the exhaust gas outlet while bypassing the turbine rotor and the boost pressure is maintained at a constant value.
The turbine rotor has to be disposed while maintaining a predetermined slight clearance between adjacent members and its circumference in order to efficiently rotate by the exhaust gas. Therefore, in the above mentioned prior turbocharger, the turbine housing and the bearing housing are constituted as a two-piece construction in order to prevent the assembling of the turbocharger from deteriorating due to the shape of the turbine rotor. Thereby, a coupling or a plate and so on are required for air-tight fastening between the turbine housing and the bearing housing. As a result, the manufacturing cost of the turbocharger is increased due to the increase in the number of the parts, and there is the danger of the exhaust gas leaking from the fastening portion between the turbine housing and the bearing housing.
Furthermore, in the above mentioned prior turbocharger, the bypass flow of the exhaust gas which is discharged toward the exhaust gas outlet via the bypass passage interferes with the main flow of the exhaust gas discharged from the turbine rotor and the exhaust pressure. Namely, the exhaust gas pressure of the outlet side of the turbine rotor is increased. As a result, the exhaust gas pressure of the inlet side of the turbine rotor is increased and thereby the efficiency of the turbocharger is decreased.